The objectives of this project are to study the function of the evolutionarily diverged but highly conserved basal H2A isoprotein, H2A.Z, in chromatin and to study the organization and expression of the gene for H2A.Z. Transcription of the H2A.Z gene is modulated by the binding of protein factors to specific DNA sequence motifs located just upstream from the core promoter of the gene. These factors either enhance or repress the rate of transcription of this gene depending on the state of cell proliferation or differentiation. This gene, therefore, provides a useful model system to study the mechanisms by which the cell adjusts the rate of transcription of housekeeping genes to the appropriate level for each growth state. In addition, several observations made on the regulation of transcription of this gene suggest that this system may be utilized both to study how cellular proteins influence the level of transcription directed by the long terminal repeat of the integrated human immunodeficiency virus and how, in turn, the HIV transactivator protein, TAT, may affect the level of transcription of cellular genes.